These studies will investigate several aspects of placental lactogen physiology including the factors affecting placental lactogen secretion in vivo and in vitro and its somatotropic and metabolic activities in fetal rats as well as in tissue culture systems. Studies from this laboratory have shown that ovine placental lactogen, but not ovine growth hormone or ovine prolactin, stimulates fetal rat liver ornithine decarboxylase (ODC) activity. All three hormones stimulate ODC activity in neonatal rat liver. Studies will be undertaken to determine the cellular basis of these differential effects. In addition, we will investigate the effects of all three hormones on fetal liver glycogen metabolism late in pregnancy. Other studies will focus on the mechanisms controlling the synthesis and secretion of human placental lactogen (hPL) by placental explants. Our studies to date show that very low extracellular calcium concentrations increase the amount of hPL secreted by placental explants suggesting that the effects of extracellular calcium on hPL secretion differ from its effects on most other hormones. Studies to delineate the basis of these effects are in progress.